The present invention generally relates to methods for mitigating the direct current (DC) content of an alternating current (AC) voltage and, more specifically, methods for mitigating the DC content of an AC voltage output from a DC-AC inverter.
The AC inverters used widely in industrial application and aerospace applications are used to convert the DC Voltage from a DC bus to an AC voltage used in motor drives. In addition, the inverters might be used for main engine start and auxiliary power unit (APU) start for an aircraft. Due to the unsymmetrical input DC voltages, the output will have a DC component superimposed on the AC voltage. This DC component may heat motors, saturate inductors, saturate transformers and may result in system tripping. Frequent system tripping may reduce the system reliability. Additionally, the unbalanced DC voltage might result in an unsymmetrical AC voltage waveform which is an unacceptable power quality problem.
AC inverters are used widely to convert the DC voltage to AC voltage which might be regulated voltage at fixed frequency (FF) to supply AC loads or variable frequency to drive AC motors. The source of the DC input used in the aircraft is usually two DC voltage sources (such as +/−270 VDC) with the mid point (neutral point) connected to the ground. This DC input voltage is assumed to be balanced (symmetrical input DC voltages), but it might not be balanced voltage. Traditional inverters use pulse width modulation (PWM) as a switching scheme to control the power electronic devices, such as power transistors or insulated-gate bipolar transistors (IGBTs). With the traditional switching PWM, the unbalanced DC input will be reflected in the AC output as a DC content which is not required. In addition, the unbalanced DC input voltage will distort the AC output voltage and increase the total harmonic distortion (THD) of the AC waveform.
As can be seen, there is a need to provide methods for mitigating DC components from an AC voltage generated from an inverter that may be supplied with an unbalanced DC input.